1. Field of Invention
This invention pertains generally to inertial sensors and the like and, more particularly to a micromachined, vibratory gyroscope and method.
2. Related Art
Vibratory gyroscopes operate by detecting Coriolis-induced motion induced by rotation of the gyroscope about a sensitive axis. When a mass is driven to oscillate along a given axis and is rotated about an axis perpendicular to the axis of vibration, a Coriolis force is generated and applied to the mass along a response axis perpendicular to the axes of vibration and rotation. The rate of rotation is measured by detecting the change in motion of the mass along the response axis caused by the Coriolis force.
Coriolis-induced forces on the vibrating masses are in phase with the velocity of the masses since the Coriolis force is proportional to the velocity. Any undesired coupling of the motion along the primary or driven axis of vibration to the response axis will give rise to a spurious motion of the masses along the response axis. This undesired coupling is generally in phase with the displacement of the masses, rather than velocity, and is often referred to as a quadrature error.
One way to sense a change in motion of a mass due to a Coriolis force is capacitive detection, which typically involves a fixed electrode and a movable electrode. In such devices, it is important to minimize motion of the movable electrode in the absence of applied rotation, i.e., any motion of the mass along the response axis which is not due to a Coriolis force. Otherwise, an undesired quadrature signal will be present, having the same frequency as the rate signal but phase shifted by 90 degrees. This quadrature signal is superimposed on the desired output signal. Although the quadrature signal can be partially rejected electronically, e.g. by the use of phase-sensitive demodulation, that tends to degrade the performance of the gyroscope.
Another source of error in a vibratory gyroscope is sensitivity to linear accelerations which displace the masses thus produce undesired outputs.
When a gyroscope is mounted on a support for a given application, any unbalanced momentum of the vibrating masses will cause part of the driving energy to be injected into the support and then potentially be coupled back to the device. Energy fed back in that manner can cause bias errors and makes the performance of the device sensitive to the mounting conditions. Using two vibratory members with equal masses and moving with equal and opposite displacements eliminates this momentum imbalance and significantly improves sensor performance.
In micromachined vibratory gyroscopes heretofore provided, the vibrating masses are generally coupled together mechanically, with a spring or equivalent structure linking the two masses and creates a force between them proportional to their relative displacement. Heretofore, it has generally been thought that such coupling is required in order to assure that the masses will oscillate at a common frequency of resonance.
Uncoupled masses generally have different resonant frequencies, which would not be conducive to a practical sensor. Using a single source of drive excitation, a system with two resonant frequencies would tend to be unstable or to operate at the resonant frequencies of one or the other of the masses.
When the masses are sufficiently coupled, the two masses will no longer oscillate with separate frequencies, but will act as a modal system. One mode of this system will generally involve a substantially differential oscillation in which the masses move with roughly equal and opposite displacements. In the event that the two masses or their supporting spring structures are not symmetrical, the two masses will undergo unequal displacements. That is an undesirable condition which couples the gyro to its support structure, making it sensitive to changes in boundary conditions.
Practical rate sensors are subject to variations due to fabrication tolerances which result in asymmetries in mass and stiffness. While the masses may be coupled, the differential mode of oscillation will not be completely symmetrical.
Another disadvantage of coupling mechanisms used for vibratory rate sensors is that many of them employ folded beam designs that increase the required substrate area and size of the device.